The high efficiency of switching power converters such as flyback converters has led to their virtual universal adaption as the battery charger for mobile devices. In a flyback converter, a primary-side controller controls the cycling of a power switch transistor that connects between the transformer's primary winding and ground. A rectified AC mains voltage drives the primary winding current when the power switch is cycled on. The rectified AC mains voltage can be several hundred volts such that it can stress the power switch transistor. To minimize the switching stress for the power switch transistor, both quasi-resonant (valley-mode switching) and zero-voltage switching techniques are known. For example, it is known to employ valley switching techniques with regard to the resonant oscillation of the drain voltage for the power switch transistor when it is cycled off. The peak voltages for the resonant oscillation can be relatively robust (as much as 200 V or higher) whereas the minimum voltages (the valleys in the resonant oscillations) are much lower. Valley-mode switching thus involves the detection or prediction of a particular valley in the resonant oscillations so that the power switch transistor may be switched on at the time of the particular valley.
Although valley-mode switching thus lowers the voltage stress on the power switch transistor, note that the valley voltages are not zero but may range from 20 V or even higher such as 60 V. This relatively high drain voltage is then discharged to ground when the power switch transistor is cycled on, which lowers efficiency. A more power-efficient alternative to valley-mode switching is zero-voltage-switching (ZVS). In ZVS operation, the leakage energy in the transformer is stored and reclaimed in a capacitor that is coupled to the drain voltage of the power switch transistor through an active clamp switch. The active clamp switch is cycled on at the peak of the resonant oscillations, whereupon the drain voltage is discharged below ground as the leakage energy is reclaimed. An ZVS architecture thus has no stressing switches at the on-time of the power switch transistor.
However, the detection of the zero-voltage switching point has so far proven to be problematic. In particular, it is conventional to calculate the circuit energy so as to estimate the needed energy to complete a half-cycle of resonant oscillation. But such an estimation relies heavily on the accuracy of the circuit parameters and is thus subject to considerable process variation. Moreover, the half-cycle estimation is lengthy and consumes substantial calculation power. The resulting inaccuracies result in either a hard turn of the power switch or waste of resonant energy and large voltage stress.
Accordingly, there is a need in the art for improved control of zero-voltage switching for switching power converters.